He will begin work in Belfast after a weekend in which tough statements from both Unionists and republicans added to what the senator once described as their "vast inventories of historical recrimination".
Expectations of an early breakthrough, or indeed any breakthrough at all, are conspicuously low both in political circles and among the public. The intense but abortive effort to find agreement in July has created an unpromising atmosphere for the new talks.
The senator, who has been involved in Northern Ireland affairs since 1994, is expected to shuttle between the various parties over the next few days. Both David Trimble's Ulster Unionists and Sinn Fein will take part in a review of the Good Friday agreement but, in a reflection of the political chill, the unionists say they will not talk directly to Sinn Fein.
Everyone expects the process to be interrupted on Thursday when Chris Patten's report on the future policing of Northern Ireland is published. Whatever its contents, its appearance is bound to lead to heated controversy.
Mr Trimble yesterday attacked both Sinn Fein and Tony Blair, referring to republicans as "those thugs" and accusing the government of appeasing evil and turning a blind eye to violence.
Sinn Fein chairman Mitchel McLaughlin in turn accused Mr Trimble's party of toughening its position, saying: "The portents for the immediate scenario of the review are negative because of the regression of the Unionist party to a harder and harder position."
There was an emotional appeal for progress from the Catholic Bishop of Down and Connor, Dr Patrick Walsh, at the west Belfast funeral of one of "the disappeared", whose body was recently recovered after 21 years. Dr Walsh said: "Let us not be treated next week to further fatuous posturing or empty point-scoring. There is far too much at stake for that - lives are at stake. It is time for the exercise of responsibility, for the exercise of leadership with courage and integrity."
Seamus Mallon of the SDLP, who in July resigned as Mr Trimble's deputy, said the talks should "go for broke", adding: "There's no point in recrimination, there's no point in each party trying to put the other in the dock. We were within touching distance of getting this executive set up."
Senator Mitchell is under no illusions about the task facing him this morning, as he tries to find grounds for accommodation where all others have so far failed. In his five years of involvement in Northern Ireland he has often commented on the welcome he has received. But he has also remarked: "People here are very nice and friendly - they're just not friendly to each other."
The book he wrote about his experiences in Belfast makes it clear that, despite his talent for exuding apparently infinite tolerance and patience, he found the months of procedural wrangling and filibustering extremely wearying. Because of this, he has already made it clear that he wants the review of the Good Friday agreement to be tightly focused and time- limited. He has the air of a man prepared to spend weeks, but not months, exploring whether a breakthrough is possible.
His job is simple: to broker a deal between Gerry Adams and David Trimble. The other parties involved hardly care what that deal might be - they simply want to see a deal done and a new government established. But achieving that means somehow resolving the conundrum of devolution and de-commissioning, and the order of priority. Tony Blair and others have made several attempts, so far to no avail.
As things stand, Sinn Fein cannot or will not deliver de-commissioning, while the Ulster Unionists will not go into government without it. Each side argues that it has the Good Friday agreement on its side; each also signals that it has gone about as far as its less flexible supporters will permit.
The Pieces In The Ulster Jigsaw
Ulster Unionist Party
Probably the most important part of the jigsaw at the moment, but also the most fissionable and unpredictable. The party is divided between those who wish to go along with the peace process and those who believe it is dangerous and should be closed down. David Trimble's strategy as leader has been to talk tough and play it long. As the months have dragged, leading party members have become more restless and undisciplined. His deputy, John Taylor, has said he will not take part in the review.
Sinn Fein, led by Gerry Adams, seems as committed as ever to the peace process, but during the summer the IRA killed one man, attempted to run guns from America and continued with "punishment" beatings and exilings. Although there is no appetite for a return to full-scale war, some in the ranks are plainly disillusioned with the process. With much ill-feeling in the air, there are no signs that republicans might soften on de-commissioning.
The loyalist paramilitaries
Still beating and exiling, like the IRA, but again no desire for a return to full-scale conflict. Some elements are heavily into drugs, while others remain committed to politics. No sign of the big groups de-commissioning though, while some smaller outfits are petrol-bombing Catholic homes.
Rev Ian Paisley
His Democratic Unionist party will not be formally going into the review but he will meet Senator Mitchell and will watch closely from the sidelines, the better to denounce any Trimble sell-out. His topping of the European poll was a sign that many Protestants prefer the old ways.
The constitutional nationalists
The Irish government and the SDLP are eager for almost any workable compromise and anxious to set up a partnership government as soon as possible. Seamus Mallon's resignation as deputy first minister, a sign of intense frustration, has increased his stature across all shades of nationalist opinion.
The middle ground
The churches, big business and the community and voluntary sectors all hope that an accommodation will be found, believing that only a cross- community administration can provide the stability needed for economic and social improvement.